This invention relates generally to wastewater treatment system, and more particularly to an outlet arrangement for the extraction of odoriferous and noxious fumes emanating from the contents of a clarifier.
Sedimentation is the process by which suspended particles heavier than water are removed by gravitational settling. Sedimentation which comes into play in the natural purification of streams and lakes is also widely used in the treatment of sewage and industrial wastes.
Systems for treating sewage and industrial wastes typically include preliminary settling tanks that collect a good part of the suspended load of impurities before the clarified effluent is discharged into receiving waters or is subjected to further treatment in secondary settling tanks. In the secondary or final settling tanks, there are collected those matters which have been converted into settleable solids or otherwise rendered settleable by biological or related treatment techniques. To improve the efficiency of the system, flocculating or precipitating agents may be added to sewage prior to sedimentation.
A settling tank or basin has four functional zones; namely, an inlet zone, a settling zone, a sludge zone and an outlet zone. For high efficiency, inlets must distribute flow and suspend matter as uniformly as possible within the tank. In the sludge zone, sludge is worked into a sump from which it is withdrawn by gravity or by a pumping action. The main concern of the present invention is with the outlet zone in which the control of outflow is usually effected by a weir attached to one or both sides of an outlet trough or launder which acts as a lateral spillway.
In an arrangement in accordance with the invention, the structure and operation of the outlet zone is such as to minimize the emission of odoriferous and noxious fumes into the atmosphere. Because incoming waste is saturated with noxious gases and easily volatilized compounds, some degree of odor control is generally necessary in a liquid waste treatment system. In sewage, the objectionable compound which predominates is hydrogen sulfide, a colorless gas which has an offensive odor. This gas is highly toxic and a strong irritant to eyes and mucous membranes. Hydrogen sulfide is almost always present in sewage, for it is the product of anaerobic decomposition by bacteria present in the sewage and in the sewage treatment system.
With settling tanks and sewage collection systems having significant detention times, the generation of hydrogen sulfide, particularly in warm weather, can be so vigorous that the gases released from the tank can create serious problems. Apart from its unpleasant odor, the presence of hydrogen sulfide in the atmosphere is hazardous to personnel in the vicinity of the tank and may cause permanent injury and even death. And since hydrogen sulfide will condense with water or concrete or metal surfaces to form sulfuric acid, this acid may severely corrode the structures.
Moreover, with sewage treatment in activated sludge units, the introduction therein of hydrogen sulfide acts to promote the growth of thirotrix bacteria, a filamentous organism which compacts poorly in the settling process, thereby degrading the performance of the facility.
Various approaches have heretofore been taken to minimize the adverse effects of hydrogen sulfide. Thus, strong oxidizing agents such as hydrogen peroxide have been added to the sewage system to inhibit the anaerobic bacteria responsible for generating hydrogen sulfide, the agents oxidizing the sulfide to form soluble, non-odoriferous orifice sulfite or sulfate compounds.
Another approach heretofore taken is to cover the settling tanks. But this approach leaves much to be desired; for when the tanks are covered with a concrete structure, the reaction of the gases with the structure then gives rise to serious corrosion problems, and the cost of complete covers is very high. Complete coverage of large tanks also creates a large enough volume over the liquid that workers can enter. Therefore, a large volume of air must be blown through this enclosed area to afford a safe working environment for personnel. This large volume of air dilutes the odoriferous compounds and makes them more difficult to treat. Thus prior attempts to solve the problem of odor control, despite their relatively high cost, have been of limited effectiveness.